GLSL Programming/Unity/Lighting Textured Surfaces

Earthrise as seen from Apollo 8.

This tutorial covers per-vertex lighting of textured surfaces.

It combines the shader code of Section “Textured Spheres” and Section “Specular Highlights” to compute lighting with a diffuse material color determined by a texture. If you haven't read those sections, this would be a very good opportunity to read them.

Texturing and Diffuse Per-Vertex Lighting

In Section “Textured Spheres”, the texture color was used as output of the fragment shader. However, it is also possible to use the texture color as any of the parameters in lighting computations, in particular the material constant ${\displaystyle k_{\text{diffuse}}}$ for diffuse reflection, which was introduced in Section “Diffuse Reflection”. It appears in the diffuse part of the Phong reflection model:

${\displaystyle I_{\text{diffuse}}=I_{\text{incoming}}\,k_{\text{diffuse}}\max(0,\mathbf {N} \cdot \mathbf {L} )}$

where this equation is used with different material constants for the three color components red, green, and blue. By using a texture to determine these material constants, they can vary over the surface.

In comparison to the per-vertex lighting in Section “Specular Highlights”, the vertex shader here computes two varying colors: diffuseColor is multiplied with the texture color in the fragment shader and specularColor is just the specular term, which shouldn't be multiplied with the texture color. This makes perfect sense but for historically reasons (i.e. older graphics hardware that was less capable) this is sometimes referred to as “separate specular color”; in fact, Unity's ShaderLab has an option called “SeparateSpecular” to activate or deactivate it.

Note that a property _Color is included, which is multiplied (component-wise) to all parts of the diffuseColor; thus, it acts as a useful color filter to tint or shade the texture color. Moreover, a property with this name is required to make the fallback shader work (see also the discussion of fallback shaders in Section “Diffuse Reflection”).

Shader "GLSL per-vertex lighting with texture" {
Properties {
_MainTex ("Texture For Diffuse Material Color", 2D) = "white" {}
_Color ("Overall Diffuse Color Filter", Color) = (1,1,1,1)
_SpecColor ("Specular Material Color", Color) = (1,1,1,1)
_Shininess ("Shininess", Float) = 10
}
Pass {
Tags { "LightMode" = "ForwardBase" }
// pass for ambient light and first light source

GLSLPROGRAM

// User-specified properties
uniform sampler2D _MainTex;
uniform vec4 _Color;
uniform vec4 _SpecColor;
uniform float _Shininess;

// The following built-in uniforms (except _LightColor0)
// are also defined in "UnityCG.glslinc",
// i.e. one could #include "UnityCG.glslinc"
uniform vec3 _WorldSpaceCameraPos;
// camera position in world space
uniform mat4 _Object2World; // model matrix
uniform mat4 _World2Object; // inverse model matrix
uniform vec4 _WorldSpaceLightPos0;
// direction to or position of light source
uniform vec4 _LightColor0;
// color of light source (from "Lighting.cginc")

varying vec3 diffuseColor;
// diffuse Phong lighting computed in the vertex shader
varying vec3 specularColor;
// specular Phong lighting computed in the vertex shader
varying vec4 textureCoordinates;

#ifdef VERTEX

void main()
{
mat4 modelMatrix = _Object2World;
mat4 modelMatrixInverse = _World2Object; // unity_Scale.w
// is unnecessary because we normalize vectors

vec3 normalDirection = normalize(vec3(
vec4(gl_Normal, 0.0) * modelMatrixInverse));
vec3 viewDirection = normalize(vec3(
vec4(_WorldSpaceCameraPos, 1.0)
- modelMatrix * gl_Vertex));
vec3 lightDirection;
float attenuation;

if (0.0 == _WorldSpaceLightPos0.w) // directional light?
{
attenuation = 1.0; // no attenuation
lightDirection = normalize(vec3(_WorldSpaceLightPos0));
}
else // point or spot light
{
vec3 vertexToLightSource = vec3(_WorldSpaceLightPos0
- modelMatrix * gl_Vertex);
float distance = length(vertexToLightSource);
attenuation = 1.0 / distance; // linear attenuation
lightDirection = normalize(vertexToLightSource);
}

vec3 ambientLighting =
vec3(gl_LightModel.ambient) * vec3(_Color);

vec3 diffuseReflection =
attenuation * vec3(_LightColor0) * vec3(_Color)
* max(0.0, dot(normalDirection, lightDirection));

vec3 specularReflection;
if (dot(normalDirection, lightDirection) < 0.0)
// light source on the wrong side?
{
specularReflection = vec3(0.0, 0.0, 0.0);
// no specular reflection
}
else // light source on the right side
{
specularReflection = attenuation * vec3(_LightColor0)
* vec3(_SpecColor) * pow(max(0.0, dot(
reflect(-lightDirection, normalDirection),
viewDirection)), _Shininess);
}

diffuseColor = ambientLighting + diffuseReflection;
specularColor = specularReflection;
textureCoordinates = gl_MultiTexCoord0;
gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
}

#endif

#ifdef FRAGMENT

void main()
{
gl_FragColor = vec4(diffuseColor
* vec3(texture2D(_MainTex, vec2(textureCoordinates)))
+ specularColor, 1.0);
}

#endif

ENDGLSL
}

Pass {
Tags { "LightMode" = "ForwardAdd" }
// pass for additional light sources
Blend One One // additive blending

GLSLPROGRAM

// User-specified properties
uniform sampler2D _MainTex;
uniform vec4 _Color;
uniform vec4 _SpecColor;
uniform float _Shininess;

// The following built-in uniforms (except _LightColor0)
// are also defined in "UnityCG.glslinc",
// i.e. one could #include "UnityCG.glslinc"
uniform vec3 _WorldSpaceCameraPos;
// camera position in world space
uniform mat4 _Object2World; // model matrix
uniform mat4 _World2Object; // inverse model matrix
uniform vec4 _WorldSpaceLightPos0;
// direction to or position of light source
uniform vec4 _LightColor0;
// color of light source (from "Lighting.cginc")

varying vec3 diffuseColor;
// diffuse Phong lighting computed in the vertex shader
varying vec3 specularColor;
// specular Phong lighting computed in the vertex shader
varying vec4 textureCoordinates;

#ifdef VERTEX

void main()
{
mat4 modelMatrix = _Object2World;
mat4 modelMatrixInverse = _World2Object; // unity_Scale.w
// is unnecessary because we normalize vectors

vec3 normalDirection = normalize(vec3(
vec4(gl_Normal, 0.0) * modelMatrixInverse));
vec3 viewDirection = normalize(vec3(
vec4(_WorldSpaceCameraPos, 1.0)
- modelMatrix * gl_Vertex));
vec3 lightDirection;
float attenuation;

if (0.0 == _WorldSpaceLightPos0.w) // directional light?
{
attenuation = 1.0; // no attenuation
lightDirection = normalize(vec3(_WorldSpaceLightPos0));
}
else // point or spot light
{
vec3 vertexToLightSource = vec3(_WorldSpaceLightPos0
- modelMatrix * gl_Vertex);
float distance = length(vertexToLightSource);
attenuation = 1.0 / distance; // linear attenuation
lightDirection = normalize(vertexToLightSource);
}

vec3 diffuseReflection =
attenuation * vec3(_LightColor0) * vec3(_Color)
* max(0.0, dot(normalDirection, lightDirection));

vec3 specularReflection;
if (dot(normalDirection, lightDirection) < 0.0)
// light source on the wrong side?
{
specularReflection = vec3(0.0, 0.0, 0.0);
// no specular reflection
}
else // light source on the right side
{
specularReflection = attenuation * vec3(_LightColor0)
* vec3(_SpecColor) * pow(max(0.0, dot(
reflect(-lightDirection, normalDirection),
viewDirection)), _Shininess);
}

diffuseColor = diffuseReflection;
specularColor = specularReflection;
textureCoordinates = gl_MultiTexCoord0;
gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
}

#endif

#ifdef FRAGMENT

void main()
{
gl_FragColor = vec4(diffuseColor
* vec3(texture2D(_MainTex, vec2(textureCoordinates)))
+ specularColor, 1.0);
}

#endif

ENDGLSL
}
}
// The definition of a fallback shader should be commented out
// during development:
// Fallback "Specular"
}


In order to assign a texture image to this shader, you should follow the steps discussed in Section “Textured Spheres”.

Summary

Congratulations, you have reached the end. We have looked at:

• How texturing and per-vertex lighting are usually combined.
• What a “separate specular color” is.